Between 1987 and the present, the Seroconversion Surveillance Project has provided the means to monitor the risk of transmission of human immunodeficiency virus (HIV) by clotting factor concentrates. One hundred and thirty-one hemophilia treatment centers in the United States are regularly contacted and data on HIV testing of patients collected. To date, 4366 (46.0%) of 9496 patients have been reported to be seropositive, and thirty-seven new seroconversions identified. Nine of these have met the CDC criteria for seroconversion while taking factor concentrate. None of the nine seroconversions were due to concentrates that had been treated to inactivate viruses and made from plasma that had been tested for HIV antibody. Our results indicate that there is a high prevalence of seropositivity in affected patient groups, but that the risk of HIV infection from currently available clotting factor concentrates is extremely low.